Is it possible to save all rendered images (in the render buffer slots) in one operation?
save all images  is for modified texture images, images that you have loaded from storage or packed in the .blend file and have been edited ,if no straight option is available  you can use a script to automate that process .
this script will save all non empty slots to the file path specified in the render tab and with the name displayed in the Image-editor and the number of the slot :  "Render_Image_slot_1.png"

import bpy

scene = bpy.context.scene
path = scene.render.filepath
ext = scene.render.file_extension
for img in bpy.data.images :
    i = 0
    if img.type == 'RENDER_RESULT' :
        print(img.name)
        for i in range(8):
            img.render_slots.active_index = i
            try :
                img.save_render(path+img.name+"_slot_%d"%i+ext, scene)
                print("slot %d saved"%i)
            except :
                print("Slot %d is empty"%i)


in case you need this more then once here is the Addon file for download install it and you'll find the entry in UV/Image-editor➞Image➞Save_all_slots, you can assign a shortcut for it.



to install :